a. Field of the Invention
This invention relates to an amine pretreatment for increasing the yield or quality of pulp obtained from an alkaline pulping process.
B. Description of the Prior Art
Until very recently, the only practical method for producing high strength chemical pulp from lignocellulosic material such as wood chips, was by the old established kraft process. In this process, lignocellulosic material is cooked in an aqueous solution containing NaOH and Na.sub.2 S. This process, however, suffers from two disadvantages, namely a relatively low pulp yield, and odorous gas emissions -- the latter arising from the use of sulfur compounds in the kraft cooking liquors. Both of these process aspects have become more critical in recent years with the rising production costs, raw material shortages, and the public pressures for a cleaner environment with less pollutive mill emissions. A number of methods for improving kraft pulp yield have been proposed, but the only processes of practical significance involve the use of sodium polysulfide as described in the text "The Pulping of Wood", R. G. MacDonald, Editor, McGraw-Hill, or H.sub.2 S as described in Vinje and Worster, U.S. Pat. No. 3,520,773, issued July 14, 1970. Both of these process changes, however, do not avoid the use of sulfur in the kraft mill and consequently kraft mill odor remains a problem.
Practical methods to avoid the use of sulfur in chemical pulp mills have long been sought after by the Pulp and Paper Industry. The soda and recently discovered two-stage soda-oxygen processes are the only processes currently available for producing high quality chemical pulp without the use of sulfur. The soda process is little used because it produces pulp of lower yield and quality compared to the kraft process. The soda-oxygen process, as described in Worster and Pudek, U.S. Pat. No. 3,691,000, issued Sept. 12, 1972, avoids these deficiencies, and produces a pulp of comparable yield and quality to the kraft process. It is a principal purpose of this invention to provide a method for pulping to even higher yields than the soda-oxygen or kraft processes without using sulfur-containing compounds.
It has also been known for many years that aliphatic amine compounds can be used as a pulping agent either alone or in combination with known alkaline pulping agents. As described in Peterson and Wise, U.S. Pat. No. 2,218,479, issued Oct. 15, 1940, a minimum of 15% by weight of the amine compound was required in the pulping liquor, with 70-100% being preferred. This apparently improves pulp yields and pulp quality.